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The London Olympics are behind us, but the question of the effects of air pollution during exercise still lingers—kind of like the smell of car exhaust in your clothes after a rush hour run.

While previous research in rats found that exercising outside in polluted air might protect your lungs, a new report by Belgian researchers showed that outdoor training could block cognitive gains.

In the study, published in Medicine & Science in Sports & Exercise, scientists found that students who trained outdoors in a small town improved their mental ability during a 12-week exercise program. Students who exercised outside in a polluted city, however, didn’t see any jump in brain power.

Researchers also found that inflammation—in the lungs and throughout the body—increased with pollution.

Does this mean you should sit on the couch during days with poor air quality? Not at all. The benefits of exercise are clear, and the previous rat study showed that exercise might protect your lungs from air pollution.